System for fusing joints

ABSTRACT

A method of fusing a joint between two bones, comprising boring a hole through one of the bones across the joint therebetween and into the other bone, placing the leading end of a screw into the hole, where the screw has a threaded region having a pitch that is larger toward a leading end of the screw and smaller toward a trailing end of the screw, and driving the screw into the bone until the threaded region spans the joint.

This application is a continuation-in-part of U.S. patent applicationSer. No. 09/034,046, filed Mar. 3, 1998, now U.S. Pat. No. 5,964,768,which is a continuation-inpart of Ser. No. 08/781,471, filed Jan. 10,1997, now U.S. Pat. No. 5,871,486, and a continuation-in-part of PCTapplication Ser. No. US94/00738, filed Jan. 19, 1994. In turn,application Ser. No. 08/781,471 is a continuation-in-part of applicationSer. No. 08/506,469, filed Jul. 25, 1995, now abandoned, which is acontinuation-in-part of Ser. No. 08/332,445, filed Oct. 31, 1994, nowU.S. Pat. No. 5,562,672, which is a continuation of application Ser. No.08/007,196, filed Jan. 21, 1993, now abandoned. All of the above patentsand applications are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to a bone screw for drawingtogether bone fragments separated by a fracture and more particularly tosuch a screw which draws the bone fragments together as a result ofdifferent-pitched threads on the screw.

BACKGROUND OF THE INVENTION

In healing bone fractures it is desirable to compress the fractures sothat the fractured surfaces are pressed against one another. In theprior ark bone screws have been used to draw the fractured surfacestogether and thereby optimize the healing process.

A number of prior art bone screws have been constructed in a fashionresembling wood screws. For example, some prior art bone screws includea threaded distal portion and a head with a relatively long unthreadedshank disposed between the head and the distal portion. A drill is usedto create a bore through the fracture and the screw is threaded into theremote bone fragment with the head of the screw compressing the nearfragment tightly against the remote bone fragment.

Other bone screws are threaded along the length thereof thus requiting afirst drill bit to create a bore in both bone fragments extending acrossthe fracture and a second bit to drill a larger bore in the near bonefragment so that the screw threads do not engage the near bone fragmentThereafter, the screw is tightened in the same manner as described abovein connection with the screw having an unthreaded shank, therebycompressing the fragments together.

The operation of two prior art headed lag screws is illustrated in FIGS.8A-10D. The operation of a lag screw A₁ with a head B₁, and a shank C₁is shown in FIGS. 8A-D. Shank C₁ of screw A₁ includes threads D₁ at thedistal end and an unthreaded region E₁ proximal to head B₁. The pitch ofthreads D₁ is constant. FIG. 8A shows screw A₁ partially engaged in abore F₁ in a near bone fragment G₁. The diameter of bore F₁ is less thanthe diameter of threads D₁ and therefore the threads engage the walls ofthe bore as the screw is twisted in. FIG. 8B shows screw A₁ as it startsthreading into a bore H₁ in a remote bone fragment I₁. At this pointthreads D₁ are engaged in both bores and moving forward at the samespeed in both fragments so no compression between the fragments isachieved. Head B₁ has reached the top of fragment G₁ in FIG. 8C, asindicated schematically by the radiating “force” lines. Since threads D₁are no longer engaged in fragment G₁, screw A₁ rotates freely in thefragment without being drawn forward therein Subsequent rotation ofscrew A₁ draws fragment I₁ further up the screw. Because head B₁prevents fragment G₁ from moving further up screw A₁, fragment I₁ isdrawn up against fragment G₁ and compression between the fragments isachieved as shown in FIG. 8D, with the head pulling down on the nearfragment and the threads pulling up on the remote fragment.

The importance of the unthreaded region of screw A₁ is illustrated inFIGS. 9A-d. A lag screw A₂ including a head B₂ and a shank C₂ is shownpartially engaged in a bore F₂ in a near fragment G₂ in FIG. 9A. ShankC₂ includes threads D₂ running the entire length with no unthreadedregion such as E₁ on screw A₁. Rotating screw A₂ causes it to be drawnthrough fragment G₂ and pass into a bore H₂ in a remote fragment I₂, asshown in FIG. 9B. Further rotation of screw A₂ brings head B₂ downagainst the upper surface of fragment G₂. See FIG. 9C. At this point,threads D₂ are still engaged in bore F₂ of fragment G₂ and theinteraction of the head on the surface of fragment G₂ impedes thefurther rotation of screw A₂. To have additional rotation, head B₂ wouldhave to be drawn down into fragment G₂ or the portion of threads D₂ infragment G₂ would have to step out. Therefore a fully threaded screw,such as screw A₂, would not be preferred for use in the fragment andbore configuration of FIGS. 9A-D.

The proper bore configuration for using screw A₂ is illustrated in FIGS.10A-D. As shown in FIG. 10A, bore F₂ in fragment G₂ is enlarged to allowthreads D₂ of screw A₂ to pass freely through the bore. Screw A₂therefore slips into bore F₂ until it reaches fragment I₂. At that pointthreads D₂ engage the walls of bore H₂ and draw screw A₂ down intofragment I₂. See FIGS. 10B-C. When head B₂ reaches the upper surface offragment G₂, further rotation causes fragment I₂ to be drawn up intocontact with fragment G₂ as shown in FIGS. 10C-D. No binding occursbetween head B₂ and reads D₂ in the near fragment because of the largebore in fragment G₂, and the screw functions as intended to draw the twofragments together.

FIGS. 11A-12D illustrate the effect of substituting headless screws inthe place of lag screws A₁ and A₂. FIG. 11A, in particular, shows aheadless screw A₃ partially installed in a bore F₃ in a near fragmentG₃. Screw A₃ includes threads D₃ extending along its entire length Thepitch of threads D₃ is constant. FIG. 11B shows screw A₃ extendingthrough fragment G₃ and just entering a bore H₃ in a remote fragment I₃.FIG. 11C shows screw A₃ advanced further into fragment I₃. It should benoted that, since the pitch of threads D₃ is constant, screw A₃ movesforward in fragments G₃ and I₃ by the same amount with each rotation. Asshown in FIG. 11D, screw A₃ will pass through both fragments withoutaltering their relative spacing or compressing them together. Thus, aheadless screw such as screw A₃ will not work to draw the fragmentstogether in the same way as lag screws A₁ and A₂.

A variation of screw A₃ is shown at A₄ in FIG. 12A. Screw A₄ includesthreads D₄ of constant pitch extending along its entire length anddiffers from screw A₃ in that it tapers from a smaller outside diameterat the leading end to a larger outside diameter at the trailing end.Screw A₄ is shown because it incorporates tapering which is one of thefeatures of the present invention, however, it is unknown whether such ascrew is found in the prior art Screw A₄ is shown partially installed ina bore F₄ in a near fragment G₄ in FIG. 12A. As screw A₄ is rotated, itmoves through fragment G₄ and into a bore H₄ in a remote fragment I₄, asshown in FIG. 12B. Subsequent rotation simply carries screw A₄ furtherinto and through fragment I₄ without any effect on the spacing betweenthe fragments. See FIGS. 12C-D. With a constant pitch Dread, such asfound on thread D₄, the taper does not facilitate compression. Tapermay, however, make a screw easier to start in a small pilot hole or evenwithout a pilot hole. The threaded portion of many wood screws followsthis general format, tapering to a sharp point, to allow installationwithout a pilot bole.

It can be seen from the above discussion that a headless screw ofconstant pitch does not achieve the desired compressive effect betweenthe two fragments as win a lag screw with a head. It is, however,possible to draw two fragments together with a headless screw if it hasvaluing pitch. FIG. 13A shows a headless screw As with threads D₅ formedalong its entire length. Such a screw is shown in U.S. Pat. No. 146,023to Russell. The pitch of threads D₅ varies from a maximum at the leadingend to a minimum at the trailing end It is expected that such a screwmoves forward upon rotation in a fragment according to the approximateaverage pitch of the threads engaged in the fragment. Screw A₅ is shownin FIG. 13A with the leading threads engaged in a bore F₅ in a nearfragment G₅. Rotation of screw A₅ causes it to move forward into andthrough fragment G₅ and into a bore H₅ in a remote fragment I₅, as shownin FIG. 13B. Additional rotation after the leading threads engagefragment I₅ causes the two fragments to be drawn together. See FIGS.13C-D. This is because the average pitch of the threads in fragment I₅is greater than the average pitch of threads in fragment G₅. Since thescrew moves forward in each fragment with each 360° rotation by anamount roughly equal to the average pitch of the threads in thatfragment, each rotation will move the screw forward further in fragmentI₅ than in fragment G₅. This effect will gradually draw the fragmentstogether as the screw moves forward. Depending on the initial spacingbetween the fragments, they can make contact either before or after thetrailing end of the screw has entered fragment G₅. It should be notedthat screw A₅, in contrast to constant pitch screws such as screws A₁and A₂, can be used to separate fragments G₅ and I₅ by simply reversingthe rotation.

One drawback of a screw such as shown in Russell is the stopping orreaming of the female threads created in the bore by the leading threadsas the trailing threads follow. Because the pitch changes along thelength of the screw, no thread exactly follows the thread directly infront of it Rather, each thread tends to cut its own new path which onlypartially overlaps the path of the thread ahead of it Thus, the trailingthreads tend to ream out the female threads in the bore made by theleading threads. This effect reduces the grip of the trailing threadsand therefore the overall compressive force available to urge thefragments together.

FIG. 14A shows a headless screw A₆, such as disclosed in U.S. Pat. No.4,175,555 to Herbert, that offers one solution to the problem of reamingof threads. As noted in the Herbert patent, bone screws having headssuffer from several disadvantages including concentrated loads beneaththe screw head and the protrusion of the screw head itself after thescrew is installed. Several other shortcomings of the standard type ofbone screw are detailed in the Herbert patent.

Screw A₆, as per Herbert, includes a shank C₆ with leading threads J₆ atthe leading end, trail threads K₆ at the trailing end and an unthreadedregion E₆ separating the leading and trailing threads. Threads J₆ and K₆each have fixed pitch, but leading threads J₆ have a larger pitch andsmaller outside diameter than trailing threads K₆. FIG. 14A showsleading threads J₆ of screw A₆ installed in a bore F₆ of a near fragmentG₆. It should be noted that threads J₆ do not engage the walls of boreF₆, the bore having been bored large enough to allow leading threads J₆to pass freely. As the screw moves forward, the leading threads engage abore H₆ in a remote fragment I₆. See FIG. 14B. The diameter of bore H₆is adapted so that leading threads J₆ engage the walls. Meanwhile, atthe trailing end of the screw, trailing threads K₆ start to engage thewalls of bore F₆, which has been bored to an appropriate diametertherefor.

As soon as trailing threads K₆ are engaged in bore F₆ and leadingthreads J₆ are engaged in bore H6, the two fragments start drawingtogether. See FIG. 14C. Further rotation of screw A₆ completes theprocess of moving the two fragments together as shown in FIG. 14D. ScrewA₆ operates on the same general principle as screw A₅, except that theaverage pitch of the threads in the remote and near fragments is simplythe pitch of the leading and ting threads, respectively. For instance,if the pitch of the leading threads is 0.2 inches and the pitch of thetrailing threads is 0.1 inches, each rotation of screw A₆ will move it0.2 inches further into fragment H₆, but only 0.1 inches further intofragment I₆, thus moving the fragments 0.1 inches closer together.

The Herbert screw overcomes at least one of the drawbacks of the Russellscrew, the reaming of female threads by subsequent threads on the screw,but at the same time suffers from a number of other disadvantages. Inthe Herbert screw, the leading threads have a smaller diameter than thetrailing threads. This is necessary to permit the leading threads topass through the relatively large bore in the near bone fragment andengage the smaller bore in the remote bone fragment. The larger trailingthreads then engage the larger bore in the near bone fragment As aresult of this arrangement, any stripping of the threads cut into thebones during installation of the screw occurs in the remote bone. If thestripping occurred in the bore in the near bone fragment, a screw havinga head thereon could still be used to compress the fracture even thoughthe near bore was stripped; however, when stripping occurs in the borein the remote bone, a standard screw with the head thereon cannot beused and another bore must be drilled.

Further, the Herbert screw must be correctly positioned, i.e., it isimperative that the fracture intersect the unthreaded central portion ofthe Herbert bone screw when the same is installed. Thus, the Herbertscrew is not suitable for treating fractures that are very near thesurface of the bone where the hole is to be drilled In addition, becausethe Herbert screw is not threaded entirely along the length thereof thepurchase obtained by the screw in the bone is not as good as with ascrew threaded along the entire length. Also, two bores of differentsizes must be drilled to install the Herbert screw rather than a singlebore.

Yet another problem with the Herbert screw is the stripping that canoccur if additional tightening occurs after the screw has drawn the bonefragments together. While the bone fragments are being drawn together,trailing threads K₆ all follow a single path through the near fragmentSimilarly, leading threads J₆ all follow a single path through theremote fragment. When, however, the bone fragments make contact, the twosets of threads can no longer move independently. Further rotation ofthe Herbert screw after contact between the fragments can cause theleading threads to strip out as they attempt to move forward through thedistal bone fragment faster than the trailing threads will allow. SeeThe Herbert Bone Screw and Its Applications in Foot Surgery, The Journalof Foot and Ankle Surgery, No. 33, Vol 4., 1994, pages 346-354 at page346, which reports on a study that found compression of 10 kg after onlytwo complete turns of the trailing threads engaged in the near bonefragment Each subsequent revolution lead to a decrease in compressiveforce. Thus, care must be taken not to over-tighten the Herbert screw.

In addition to drawing two bone fragments together to repair fractures,it is sometimes desirable to draw together two bones for fusing the sametogether in connection with arthrodesis of the interphalangeal joints.This procedure is sometimes indicated with symptoms of pain orinstability in the finger joints. The purpose is to immobilize and drawtogether adjacent bones across a joint to cause them to fuses togetherthereby preventing further movement at the joint.

In one prior art procedure for immobilizing the distal interphalangealjoint (DIP), axial bores are drilled in the particular surfaces of thedistal and proximal phalanges. The bore in the distal bone issufficiently large to receive without threading a screw which isinserted therein via an incision in the tip of the finger. The screwthreadably engages the bore in the proximal bone and when the screw headis tightened against the distal end of the distal bone, the two bonesare compressed together. After several weeks, the bones fuse together. Asecond procedure to remove the screw must be performed because the headof the screw will cause discomfort in the finger pad if the screw is notremoved.

This procedure is undesirable because it requires two separatesurgeries. Katzman, et al., Use of a Herbert Screw for InterphalangealJoint Arthrodesis, Clinical Orthopedics and Related Research, No. 296pages 127-132 (November 1993), describes use of the screw disclosed inthe Herbert patent in procedures for interphalangeal joint arthrodesis.

Many of the above-discussed disadvantages associated with using aHerbert screw to compress a fracture are also present when the Herbertscrew is used for interphalangeal joint arthrodesis.

It would be desirable to provide a headless bone screw which overcomesthe disadvantages associated with the Herbert bone screw, as well asother prior art bone screws.

SUMMARY OF THE INVENTION

A bone screw for drawing together bone fragments separated by a fractureincludes a root portion having a leading end and a trailing end. Theleading end has a smaller diameter than the trailing end. A screw threadis formed on the root portion between the leading and trailing ends andhas a pitch which varies along the length thereof, having a larger pitchnear the leading end and a smaller pitch near the filing end. The threadis adapted to thread in the cancellous material of the respective bonefragments to be joined by the screw. Means are provided on the trailingend of the root portion to accommodate a tool for driving the screw. Thepresent invention also contemplates a method for drawing together bonefragments separated by a fracture.

The foregoing and other objects, features and advantages of theinvention will become more readily apparent from the following detaileddescription of a preferred embodiment which proceeds with reference tothe drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged side elevation view of a bone screw constructed inaccordance with the present invention;

FIG. 1A is a view of the screw of FIG. 1 shown partially in crosssection;

FIG. 2 is an end view of the bone screw of FIG. 1;

FIG. 3 is a drawing illustrating the outside diameter of the screw;

FIG. 4 is a drawing illustrating the diameter of the root portion of thescrew;

FIG. 5 is a cross-sectional view of a bone screw constructed inaccordance with the present invention installed in a bone to draw afracture together;

FIG. 6 is a side elevation view of a bone screw constructed inaccordance with the present invention which may be used forinterphalangeal joint arthrodesis;

FIG. 7 is a view of the bone screw of FIG. 5 installed in a distalinterphalangeal joint with the bones forming the joint as shown incross-section;

FIGS. 8A-14D show the operation of various screws to compress two bonefragments together,

FIGS. 15A-D show the operation of a screw constructed according to analternative embodiment of the present invention to compress two bonefragments together;

FIGS. 16A-B are detailed views of the screw shown in FIGS. 15C and 15D,respectively;

FIG. 17A is a side elevation view of a bone screw constructed accordingto an alternative embodiment of the present invention;

FIG. 17B is a representation of the side profile of a root portion ofthe screw of FIG. 17A;

FIG. 17C is a representation of the outside diameter of the screw ofFIG. 7A;

FIG. 18A is a side elevation view of a bone screw constructed accordingto a fourth embodiment of the present invention;

FIG. 18B is a representation of the side profile of a root portion ofthe screw of FIG. 18A;

FIG. 18C is a representation of the outside diameter of the screw ofFIG. 8A;

FIG. 19 is an enlarged side elevation view of a bone screw constructedin accordance with an alternative embodiment of the present invention;

FIG. 19A is a view of the screw of FIG. 19 shown partially in crosssection;

FIG. 20 is an end view of the bone screw of FIG. 19;

FIG. 21a illustrates the outside diameter and root profile of analternative embodiment of the present invention; and

FIG. 21b is an elevational view of the screw of FIG. 21a.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Indicated generally at 10 in FIGS. 1 and 1A is a bone screw constructedin accordance with the present invention Bone screw 10 is centered on alongitudinal axis 11. The length of screw 10 as measured along axis 11is 0.394 inches in the present embodiment of the invention. The bonescrew includes a root portion 12 having a continuous screw thread 14formed thereon.

Root portion 12 includes a leading end 16 and a trail end 18. As canbest be seen in FIG. 4, the diameter of leading end 16 is less than thediameter of trailing end 18. Also in FIG. 4, it can be seen that rootportion 12 tapers between trailing end 18 and leading end 16. A 45°bevel 20, in FIGS. 1 and 1A, is formed on trailing end 18. In thepresent embodiment of the invention, tailing end 18 has a diameter ofapproximately 0.092 inches. A frusto-conical nose portion 22 is formedon leading end 16 of root portion 12.

Screw thread 14 extends continuously between nose portion 22 and bevel20. As can be seen in FIGS. 2 and 3, a tailing thread 24 has a crestheight, i.e., the distance between axis 11 and a crest 26 of trailingthread 24, which varies so as to form a substantially 45° angle,illustrated as angle 28 in FIG. 3, between the outside diameter of crest24 and axis 11.

A similarly tapering leading thread 30 also has a crest 32 which variesin height over a first partial time of screw thread 14 so as to form anangle of substantially 45° with axis 11 as illustrated in FIG. 3.

The crest of screw thread 14 between trailing and leading threads 24, 30respectively, varies in height along the length of thread 14. In thepresent embodiment of the invention, the outside diameter defined by thecrest of thread 14 between the leading and trailing threads forms anangle 34, in FIG. 3, of approximately 1.43° with respect to an axis 35extending from the radially outermost portion of thread 14 parallel toaxis 11. In the present embodiment of the invention, the diameter of theradially outermost portion of thread 14 is approximately 0.138 inches.

The pitch of thread 14, i.e., the distance from one point on the threadto the corresponding point on an adjacent thread measured parallel toaxis 11, decreases between the leading and trailing ends of the screw.It should be noted that the term pitch is also sometimes used to referto the number of threads per unit length, i.e., 20 threads per inch Thisalternative definition is simply the inverse of the definition chosenfor use in this application. The distinction is important to rememberfor proper understanding of the subsequent description because the screwof the present invention relies on varying pitch to achieve itsfunction.

In the embodiment of the invention shown in FIGS. 1 and 1A, the distancebetween the uppermost portion of crest 32 and a corresponding crestportion 36 is 0.04964 inches. The distance between the uppermost portionof crest 26 and a corresponding crest portion 38 is 0.04748 inches. Inthe present embodiment of the invention, the pitch change per revolutionis approximately 0.00036 inches.

The pitch depth, i.e., the distance between the crest and the radiallyouter surface of root portion 12 similarly varies along the length ofthe screw. In the present embodiment of the invention, the pitch depthwhere leading thread 30 joins the remainder of screw thread 14 isapproximately 0.0302 inches. The pitch depth where trailing thread 24joins the remainder of thread 14 is approximately 0.0240 inches.

The decrease in pitch depth between the leading end and trailing end ofthe screw can be seen by comparing FIG. 3 and FIG. 4 wherein rootportion 12 tapers more sharply from the trailing to the leading end ofthe screw than does the change in crest height as shown in FIG. 3. Inthe present embodiment of the invention, the outside diameter of rootportion 12 between leading and talk ends, 16, 18, respectively, forms anangle 40, in FIG. 4, of approximately 2.5° with respect to an axis 42extending from the radially outermost portion of trailing end 18parallel to axis 11.

A hex socket 44 is formed on the trading end of screw 10 to accommodatea driver as will be hereinafter further explained in connection with adescription of the procedure in which the screw is used to draw opposingfragments of a fractured bone together.

An alternative embodiment of the screw of the present invention is showngenerally at 410 in FIGS. 19 and 19A Screw 410 includes a root portion412 on which is formed a thread 414. Thread 414 extends from a leadingend 416 to a trailing end 418 and includes a land 474. The pitch ofthread 414 at the leading end is 0.055 inches and the pitch at the tangend is 0.035 inches. The land varies from 0.010 inches to 0.004 inchesovert the same range. Thread 414 includes a cutting flute 415 near theleading end to facilitate the cutting of female threads as the screw isinstalled. Both the outside diameter of thread 414 and root 412 taperfrom a smaller value at the leading end to a larger value at the bailingend. See FIGS. 21-22. The root diameter tapers from 0.062 inches to0.122 inches, while the outside diameter tapers from 0.130 inches to0.156 inches. The length of screw 410 is 0.689 inches.

Screw 410 also includes an axial bore 425 which extends from the leadingend to the trailing end. Bore 425 is adapted to receive a stiff guidewire, not shown, which facilitates installation of screw 410. A hexsocket 444 is formed at the trailing end to allow the screw to be drivenby an hex wrench See FIG. 20.

Turning now to FIG. 5, illustrated therein is a fracture 46 whichseparates adjacent bone fragments 48, 50. Screw 10 is illustratedinstalled in a bore 52 which extends through bone fragments 48, 50across fracture 46.

In installing screw 10, a surgeon first drills bore 52 across bonefragments 48, 50 as shown The bit may be a conventional cylindrical bonebit or may comprise a bit having a slight taper from the leading to thetrailing end thereof. Thereafter, the surgeon inserts a tool (not shown)having a hex driver extending therefrom which is connectable to hexsocket 44 for screwing screw 10 into bore 52. Bore 52 is of a size tojust receive leading end 16 of screw 10. As soon as nose portion 22 isreceived within the bore, torque is applied using the tool inserted intohex socket 44 thereby causing leading thread 30 to cut into the boneadjacent bore 52.

In the view of FIG. 5, screw 10 is hatched to show the path cut byleading thread 30 after screw 10 is installed in the positionillustrated in FIG. 5. The path of thread 30 is depicted using hatching,like hatching 54, 56, 58 which indicates the position of the path cut byleading thread 30 relative to succeeding threads of the screw. Hatching60 depicts the actual position of the thread on screw 10 and root 12. Itis to be appreciated that hatching 54, 60 are not used in FIG. 5 todepict different structure, which is unitary as illustrated in FIG. 1,but to depict relative positions of the path cut by leading thread 30 inthe actual position of subsequent threads in the installed screw.

Because of the decreasing pitch along the length of the screw, eachsuccessive thread received in the path cut by thread 30 exerts pressureagainst the right side (as viewed in FIG. 5) of the path cut by thread30 thereby tending to compress the bone along the length of the screw.As can be seen in FIG. 5, by the time the screw is fully installed,trailing thread 24 compresses a substantial amount of bone when it isreceived in the path cut by thread 30. This tends to draw bone fnents48, 50 tightly together across fracture 46 thereby promoting healing ofte fracture.

As can be appreciated from the view of FIG. 5, the thread taper isimportant for two reasons. First, each succeeding portion of the threadis spaced further radially outwardly as a result of the taper andtherefore the outer portion of each thread (that portion closelyadjacent the crest) cuts into new bone which was not cut by thepreceding thread This provides a much better purchase than would athread having a continuously varying pitch with constant diameter. Insuch a configuration, each succeeding thread cuts additional bone withinthe generally cylindrical volume defined by the outside diameter of thethreads. The outer portion of each thread (that portion closely a4jacentthe crest) therefore cuts into bone uncut by the preceding thread.

The tapered root is also advantageous in that the radially outer surfaceof the root, i.e., that portion between adjacent threads, is tightlyurged against uncut bone defining the wall of bore 52. It is desirableto maximize the surface area of screw 10 urged against adjacent bone,rather than a space cut by a thread, to increase purchase of the screw.

The details of the operation of the screw of the present invention, ascurrently understood, may be better appreciated by examination of FIGS.15A-D and FIGS. 16A-B and the following description FIGS. 15A-Dillustrate the operation a screw 310 to draw together and join bonefragments 348 and 350. FIG. 15A shows screw 310 partially installed in abore 349 in bone fragment 348. Screw 310 is shown just entering a bore351 in bone fragment 350 in FIG. 15B. Subsequent rotation of screw 310starts the process of drawing the bone fragments together as shown inFIGS. 15C-D.

FIG. 16A shows the interaction of a thread 314 in bores 349 and 351 whenscrew 310 is positioned therein as shown in FIG. 15C. In FIG. 16A aleading end 316 of screw 310 is engaged in bore 349. Each revolution ofthe thread 314 is labelled for reference in the subsequent discussion,from thread T1 at the leading end to thread T23 at the trailing end.

As the screw moves through bone rents 348 and 350, thread 314 will cut amating female thread 353. However, because the pitch of thread 314changes along the length of the screw, female thread 353 will notprecisely match thread 314 of screw 310 along its entire length. Inparticular, since subsequent threads will not track in the same path asthe preceding threads, a pattern of leading gaps 355 and trailing gaps357 will evolve between female thread 353 and screw thread 314 as thescrew moves forward in the bores.

The screw will move forward in the bone fragment with rotation at a ratethat is a function of the competing forces from all of the threadsengaged in the bore. The rate will correspond to an effective pitch ofthe threads in the bore and will be equal to the pitch of the screw atan effective pitch point 359 along the portion of the screw engaged inthe fragment. As more of the screw enters the bore, the effective pitchpoint will move back along the screw and further into the bone fragment.Once the screw extends completely through the bone fragment, thelocation of the effective pitch point will stabilize at a relativelyconstant location in the bone fragment, simply moving back along thescrew at the rate the screw moves forward in the bore. The threads aheadof the effective pitch point, which will be referred to as the pllingthreads 371, will have greater pitch than the effective pitch.Similarly, the threads behind the effective pitch poit, or draggingthreads 373, will have a pitch that is smaller than the effective pitch.In FIG. 16A the pulling threads in fragment 348 are T₁-T₄ and thedragging threads are T₅ and T₆.

Each rotation of the screw will move it forward in fragment 348 by anamount corresponding to the present value of the effective pitch. InFIG. 16A the effective pitch will be equal to the pitch of thread 314between threads T₄ and T₅. Starting at the leading end, thread T₁. willalways be cutting a new thread path in the fragment, so no gap will formaround it. Thread T₂, however, will attempt to follow the track ofthread T₁ in fragment 348, which would carry it forward by an amountequal to the pitch between thread T₁ and T₂. Since, however, the screwwill only move forward by the effective pitch, i.e., the pitch betweenthreads T₄ and T₅, thread T₂ can only move forward by the same amountThis causes thread T₂ to pull back against the surrounding bone andcreates a leading gap in front that thread. Similarly, thread T₃ willattempt to move into the position of thread T₂, but will be held backfrom moving as far forward as its pitch would indicate, thus creating aleading gap as thread T₃ is pulled back against the surrounding bone.Behind the effective pitch point, thread T₆ will attempt to move intothe prior position of thread T₅, but will be dragged forward somewhat,leaving a trailing gap.

The pattern of leading and trading gaps created by screw 310 in bonefragment 350 is also shown in FIG. 16A. Bone fragment 350 includesleading gaps 361 and trailing gaps 363 similar to those found in bonefiagment 348. However, because more of the screw has moved through bonefragment 350, the gaps have evolved to a greater extent The earlierposition of screw 310 in fragment 350 is shown in dotted lines in FIG.16A to illustrate the evolution of the threads as the screw movesforward.

In the earlier position of screw 310, the effective pitch point falls atapproximately thread T₈. With the screw positioned as shown, theeffective pitch point is at approximately thread T₁₆, the screw havingcompleted approximately 8 revolutions between the two positions. Thecurrent and prior screw positions are aligned at effective pitch point367 in fragment 350 based on the assumption that thread 314 will trackthrough this point uniformly. The evolution of the position of threadsbehind and ahead of the effective pitch point can thus be seen bycomparing the prior position with the current position.

Leading gaps 361 have a sloping upper surface 365, which is a result ofthe gradual expansion of the outside diameter of thread 314 toward thetrailing end of the screw. Upper surface 365 represents a line from theprior position of the thread to the position as shown. As thread 314 ata given point in the bone fragment is held back, it simultaneouslyexpands in diameter. This effect prevents thread 314 from completelyreaming out the female thread in the bone fragment, as discussed above.Without the taper, sloping upper surface 365 would be flat and as soonas the width of the gap grew to equal the spacing between the threads,there would be no purchase left for subsequent threads along a portionof the bore.

Once the leading end of screw 310 has passed through bone Went 351 theeffective pitch point remains at a relatively constant position alongthe bore for the remainder of the screw. If the pitch change perrevolution is dP and the effective pitch points are separated by Nthreads, then the bone fragments will be drawn together by a distance Ntimes dP for every revolution of the screw. In screw 310, dP=0.0008inches and the effective pitch points are separated by approximately 11threads, therefore the gap between the bone fragments will close byabout 0.009 inches per revolution.

It is thought that the effective pitch point will be somewhat behind thegeometric middle of the portion of the screw engaged in the bore asshown in FIG. 16A. Because bone becomes less dense near the center inthe cancellous portion, the threads nearer to the surface in the cortexare expected to have greater effect. A₁so, the lads nearer the surfaceare of larger diameter because of the taper in the outside diameter ofthe thread.

The other factor tending to cause the pitch point to be closer to thesurface of the bone relates to balancing the amount of bone displaced asthe leading and trailing gaps are formed. As shown in FIG. 16A, thepulling threads 371, which have pitch greater than the effective pitch,are held back from moving as far forward with each rotation as theirpitch would indicate. Likewise, dragging threads 373 are drawn forwardfaster tand their pitch would dictate. This effect creates leading gaps355 in front of pulling threads 371 as 20 they pull against thesurrounding bone. Similarly, trailing gaps 357 form behind draggingthreads 373 as they are dragged forward through the surrounding bone.

Since the leading and tailing gaps are formed in opposition to oneanother, it is reasonable to assume that they will evolve at arelatively balanced rate. Combining this assumption with the fact thatthe effective pitch point is constantly moving forward in the bonefragment as the screw enters, suggests that the effective pitch pointwill be behind the geometric middle of the portion of the screw in thebone fragment. Because the effective pitch point is moving forward inthe bone fragment by approximately one-half the pitch change perrevolution, the dragging threads will be dragged forward byapproximately an extra one-half the pitch change per revolution for eachrevolution of the screw. The fact that the effective pitch point ismoving forward means that the pulling threads are not held back as muchas would be the case if the effective pitch point remained constant. Ifthe movement of the two tbread regions through the bone are balanced,then the effective pitch point will not move forward in the bonefragment as rapidly as would otherwise be expected and the effectivepitch point will lie behind the geometric middle.

FIG. 16B shows how the pattern of gaps changes once the two bonefragments have been drawn together. After the bone fragments meet, thepattern of gaps starts to evolve toward that found in a single fragmentIn particular, gaps form or increase on the leading side of all of thepulling threads ahead of an effective combined pitch point 369, and onthe traling side of all the dragging reads behind the effective combinedpitch point. Near the joint between the fragments, the gaps willgenerally transition from leading to trailing and vice versa, becausethe dragging threads in fragment 348 near the joint are converted topulling threads after the joint closes. The pulling threads in ftagment350 likewise become dragging hireads after the fragments meet.

Rotation of screw 310 after the bone fragments have come together tendsto increase the pressure in the joint between them. Additional rotationcan be used to set the depth of the screw as desired. Since the outsidediameter of the thread tapers, as described above, the screw can bedriven in until the trailing end is below the surface of the bonewithout danger of stripping the female thread formed by the precedingthreads, even if the bone fragments first meet with the trailing endprotruding substantially. This is because subsequent thiads expand andcut into some new bone even as they partially ream the female threadsleft by preceding threads on the screw. This is in contrast to theHerbert screw, where, as discussed above, additional tightening afterthe fragments have come together can strip out the threads in the distalfagment and reduce compression. Since it is important in the preferredapplication of the present invention to have the trailing end of thescrew below the surface of the bone, this is an important feature andadvantage over prior art screws.

The tolerance in the screw of the present invention to furthertightening after the fragments have come together is also importantbecause it simplifies the installation process by eliminating the dangerof over-tightening that must be guarded against when using the Herbertscrew.

Turning now to FIG. 6, indicated generally at 62 is a second embodimentof a bone screw constructed in accordance with the present invention.Bone screw 62 is sized and constructed for use in connection withintelphalangeal joint arthrodesis. Screw 62 includes a tapered root 64having a thread 65 formed thereon from a leading end 63 to a trailingend 67, a substantially cylindrical leading extension 66 joined to theleading end and a substantially cylindrical tailing extension 68 joinedto the tailing end. The diameter of leading extension 66 is slightlylarger than root 64 at leading end 63, while the diameter of railingextension 68 is slihe* smaller than root 64 at trailing end 67. Thetrailing extension 68 includes a hex socket (not visible), like hexsocket 44 in FIG. 1A, formed on an end surface 70 thereof Leadingextension 66 includes a tapered nose 72 formed on the forward endthereof In the present embodiment of the invention, screw 62 is 1.259inches in length with the threaded portion being 0.630 inches long andthe diameter of leading extension 66 being 0.05 inches. The trailingextension diameter is 0.100 inches. As is the case with the previouslydescribed embodiment, the pitch of thread 65 decreases between theleading and trailing ends. in the embodiment of FIG. 6, a land 74 isformed in the crest of thread 65 and decreases in width between theleading and trailing ends of the screw.

Turning now to FIG. 7, a distal phalanx 76 comprises the outermost boneof one of the four fingers. A proximal phalanx 78 is adjacent theretowith a distal interphalangeal (DIP) joint 80 being formed therebetween.

The joint includes a pair of articular surfaces 82, 84 which have beenflattened in accordance with a known technique for immobilizing DIPjoint 80. Bores 86, 88 are drilled into each of phalanxes 76, 78 fromarticular surfaces 82, 84, respectively. Thereafter the bones arerepositioned as shown in FIG. 7 and screw 62 is driven into the distalend of the bore in phalanx 76 until the screw is positioned as shown inFIG. 7.

Screw 62 thus compresses across joint 80 even though it has a relativelysmall diameter, which is critical in DIP joint arhirodesis because ofthe small diameter of the bones involved. Screw 62 also has sufficientlength, due to the leading and traling extensions 66, 68, to providestability while the bones are fusing. Because the screw is entirelyreceived within the bones, i.e., there is no protrusion from the screw,it can remain implanted and thus a second procedure to remove the boneis not necessary.

A third embodiment of a screw constructed according to the presentinvention is shown generally at 110 in FIG. 17A. Screw 110 includes aroot portion 112 on which is formed a continuous screw thread 114 andassociated land 174. Screw 110 includes a leading end 116 and a tailingend 118. Leading cutting flutes 115 are fonned in thread 114 nearleading end 116 to help the thread self tap into the bone. A series oftig cutting flutes 117 are formed in thread 114 along the sides of thescrew toward the trailing end. Trailing cutting flutes 117 facilitateistallation and removal of the screw by helping to cut a thread path inthe bone. Screw 110 may be formed with two sets of halig cutting flutes,one oriented to cut female threads upon insertion and another orientedto cut female threads upon removal of the screw, thus easing bothinstallation and extraction. A hex socket 144 is formed in the trailingend of screw 110 to receive a drive tool.

Screw 110 is forned with a variable pitch portion 119 and a constantpitch portion 121. Variable pitch portion 119 extends from leading end116 back toward traling end 118 for about 70 percent of the length theof the screw. The length of the screw is 0.961 inches. It should benoted that screw 110 does not include a bevel at the trailing end asformed on screw 10 and shown at 20 in FIG. 1A The bevel was elnninatedin screw 110 to provide additional structal support around hex socket144 which is used for drving the screw.

Variable pitch portion 119 of screw 110 is formed according to thepreviously described construction of screw 10. In particular, the pitchof thread 114 is largest at leading end 116 and decreases over variablepitch portion 119 back toward traig end 118. The pitch starts at 0.050inches and decreases to 0.0365 inches at the trang end of the variablepitch portion. As shown in FIG. 17B, root portion 112 tapers outwardfrom leading end toward trailing end over variable pitch portion 119with an angle 140 of 1.93° relative to the longitudinal axis of thescrew. The diameter of the root portion is 0.032 inches at the leadingend and 0.091 inches at the trailing end The outside diameter of threadincreases over the same region at an angle 134 of 1.0°. See FIG. 17C.The outside diameter of the thread at the leading end is 0.077 inchesand 0.1 inches at the traig end.

The construction of constant pitch portion 121 is considerably differentfrom that of variable pitch portion 119. The pitch and outside diameterof thread 114 are constant over the section of the screw formingconstant pitch portion 121. Root portion 112 continues to taper outwardrelative to the axs of the screw but at a lesser angle 127 of 1.57° overthe constant pitch portion. The width of land 174, i.e., the flat at thecrest of the thread, which decreases from the leading end over thevariable pitch portion, increases over the length of the constant pitchportion toward the trailing end. Land 174 starts at the leading end at0.008 inches and decreases to 0.002 inches at the end of the variablepitch region. Land 174 starts to increase again moving back over theconstant pitch portion, reaching a value of 0.006-0.007 inches at thetraling end.

The constant pitch portion at the rear of screw 110 allows constructionof a longer screw without the commensurate increase in diameter thatwould occur by extending the structure of the variable pitch portion.This is important where the screw is to be used in small bones thatcannot accept a larger bore, but which require a longer screw. A longerscrew may be required to reach deeper fractures or for use in fusing twobones together. Screw 110 is particularly suitable for use in distalinterphalangeal fusions in the hand as described above.

A fourth embodiment of a screw constructed according to the presentinvention is shown at 210 in FIG. 18A. Screw 210 is generally similar toscrew 110 of FIG. 17A, and includes a root portion 212, a thread 214, aleading end 216 and a trailing end 218. Screw 210 also includes avariable pitch portion 219 and a constant pitch portion 221. See FIG.18B. The diameter of root portion 212 tapers at an angle 240 of 2.29°from 0.050 inches at the leading end to 0.106 inches at the trailingend. The outside diameter of thread 214 tapers at an angle 234 of 1.20from 0.110 inches to 0.140 inches over the same range. The overalllength of screw 210 is 0.787 inches.

The principal difference between screws 110 and 210 is found in theconstant pitch portions. In screw 210, neither the root portion nor theoutside diameter of the thread is tapered in the constant pitch region.See FIGS. 18B-C. Screw 210 is designed, like screw 110, to haveadditional length without additional thickness. If additional length isdesired, it is possible to form screw 210, or screw 110, with leadingand/or trailing extensions such as found on screw 62 in FIG. 6.

Thread 214 on screw 210 includes a land 274. Land 274 starts at amaximum of 0.007 inches at the leading end and decreases to 0.003 inchesat the tailing end In contrast to screw 110, land 274 does not increaseover the constant pitch portion. Thread 214 also includes leadingcutting flutes 215 and trailing cutting flutes 217 to facilitateinstallation and removal.

Screw 210 also varies from screw 110 in that it includes an axial bore225. Axial bore 225 permits screw 210 to be guided into the bone on astiff wire to facilitate positioning and prevent the screw fromwandering off axis as it is driven in.

It should be noted that the length, nwnber of threads, pitch, pitchchange per revolution and the various diameters are not critical to thepresent invention and can be varied without departing from the spirit ofthe invention. Such parameters are chosen to suit the particular use towhich the screw is applied.

A screw according to the present invention particularly adapted for usein ankle fusions is shown genelaly at 410 in FIGS. 21a-b. Screw 410includes a root portion 412 that tapers at a constant rate from aleading end 414 to a trailing end 416. In the prefenred embodiment theroot has a length of 2.383-inches and tapers from a radius of0.184-inches near the trailing end to a radius of 0.098-inches near theleading end.

A screw thread 418 is formed on root portion 412 and extends from theleading end to the traling end thereof. Thread 418 has a thread crest420 at its radial outermost edge. As with the previously describedembodiments, the thread is terminated at the leading end and trailingend with a 45-degree taper. Thread 418 has a pitch measured betweenconsecutive thread crests which varies between a larger value near theleading end to a smaller value near the trailing end. Preferably, thepitch changes uniformly between the ends from a value of 0.097-inches atthe leading end to a value of 0.066-inches at the trailing end.

In contrast to the previously described screws, screw 410 has a guidetaper 422 at the leading end of the root portion. The guide taper has ataper angle of approxately 15-degrees and serves to help maintain theleading end of the screw centered in the pilot hole in the bone in whichit is installed. The guide taper extends along the root portion backfrom the 45-degree taper for a distance of 0.129-inches.

Screw 410 has a region 424 of constant outside diameter that extendsback from the guide taper for a length of 0.090-inches with a diameterof 0.205-inches. A second region 426 of constant diameter is disposedadjacent the trailing end of the screw with a diameter of 0.256-inchesfor a length of 0.197-inches. Provision of regions 424 and 426 allowsscrew 410 to have a long length while reducing the amount of taper thatwould otherwise be required. It is important to maintain the radius aslarge as possible near the lead end to obtain adequate grip in thisregion. This is particularly important in the preferred application forscrew 410 of ankle fusions because the amount of screw 410 engaged inthe tibia may be limited. It is likewise important not to make theradius at the trailing end any larger than necessary to minimize thesize of the hole required. The region of constant diameter at thetrailing end is also important because it provides a region for grippingthe screw duing manufacture. Between the regions of constant diameter isa central region 428 in which the pitch and diameter of the screw changetogether. The central region has a length of 1.870-inches in thepreferred embodiment.

A significant difference between screw 410 and the previously describedembodiment lies in the formation of the threads. in particular, in thepreviously described embodiments, the screw thread is cut with a toolwith a flat face and outwardly sloping sides. In the previousembodiments, the width of the face determines the spacing between thethreads on the root portion, which was therefore constant along thelength of the screw. By pulling the tool back from the axis of the screwand adjusting the pitch properly, the thread can be cut with a varyingpitch and depth. However, with each pass of the tool along the screw,the tool follows the same longitudinal path in the thread but simplycuts closer to the root portion. The land at the crest is also increasednear the leading end to allow for additional pitch gain near the leadingend while maintaining a decreasing outside radius.

In screw 410, in contrast, the longitudinal position of the tool alongthe root portion is changed from pass to pass as the screw is beingturned. In particular, in one pass down the screw thread, the toolfollows a first path. In a subsequent pass the tool is shiftedlongitudinally along the screw slightly at the same depth to increasethe width of the inter-thread distance 428 on the root toward theleading end. Cutting the thread in this fashion allows a sharper threadto be produced while sfill obtaining the desired outside diameter taperand pitch variation. Sharper thread is beneficial because it leaves asmaller track in the bone which leaves more bone for subsequent threadsto grip and makes the screw easier to dive in during installation. Aswith previously descnrbed embodiments, it is important that the radiusand depth of the threads near the leading end be sufficient to provide agrip on the bone which is comparable to the grip of the threads near thetrailing end of the screw.

It should be understood that screw 410 could be manufactured in avariety of lengths to accommodate different size patients. Moreover, forshorter screws, the region of constant outside diameter near the leadingend may be eliminated without unduly compromising the grip of theleading threads. It should also be noted that shorter screws willtypically taper at a greater angle.

In the actual fusion, a hole is drilled up from the heel through thecalcaneous and talus and into the distal end of the tibia. The screw isthen driven into the hole to draw the tree bones together. With time,the pressure generated by the screw leads to fusion of the bones. Thepresent screw is advantageous for this operation because it can bemounted sub-surface since it does not have a head. Furthermore, thescrew offers excellent grip and controllable compression when comparedwith standard lag screws.

Although not shown in FIGS. 21a-b, screw 410 preferably is cannulatd toprovide improved stability during installation.

While the invention has been disclosed in its preferred form thespecific embodiments thereof as disclosed and illustrated herein are notto be considered in a limiting sense because numerous variations arepossible. Applicant regards the subject matter of his invention toinclude all novel and non-obvious combinations and subcombinations ofthe various elements, features, finctions, and/or properties disclosedherein. No single feature, finction, element, or properly of thedisclosed embodiments is essential. The following claims define certaincombinations and subcombinations which are regarded as novel andnon-obvious. Other combinations and subcombinations of features,fimctions, elements, and/or properties may be claimed through amendmentof the present claims or presentation of new claims in this or a relatedapplication. Such claims, whether they are broader, narrower, or equalin scope to the original claims, also are regarded as included withinthe subject matter of applicant's invention.

We claim:
 1. A method of fusing a joint between two bones, comprising:boring a hole through one of the bones across the joint therebetween andinto the other bone; placing the leading end of a screw into the hole,where the screw has a single continuous threaded region having a pitchthat is larger toward a leading end of the screw and smaller toward atrailing end of the screw; and driving the screw into the bone until thetreaded region spans the joint.
 2. The method of claim 1, wherein thejoint is an ankle.
 3. The method of claim 1, wherein the joint isphalangeal joint.
 4. The method of claim 1, wherein the screw includesan unthreaded leading end.
 5. The method of claim 1, wherein the screwincludes an unthreaded trailing end.
 6. The method of claim 1, whereinthe thread has a depth that is greater near the leading end of the screwand smaller toward the trailing end of the screw.
 7. The method of claim1, wherein the thread has an outside diameter that tapers over at leastthe first few thread revolutions.
 8. The method of claim 1, wherein thethread has a region of constant diameter toward the trailing end.
 9. Themethod of claim 1, wherein the pitch is approximately fifty-percentlarger toward the leading end than toward the trailing end.
 10. Themethod of claim 1, wherein the outside diameter of the threads has aconical truncation at the leading end of the threaded region.
 11. Themethod of claim 1, wherein the thread has a land, and the land is largernear the leading end than near the trailing end.